[Enh]: Session Context Claim Forwarding

See also: https://github.com/Azure/data-api-builder/issues/3078

## Config (Data Source Options Section)

```json
{
"data-source": {
"database-type": "mssql",
"connection-string": "@env('SQL_CONN')",
"options": {
"set-session-context": true,
"session-context-claims": [ // new
"sub",
"preferred_username",
"realm_access.roles",
"https://example.com/tenant"
]
}
}
}
```

## What?

Add `session-context-claims` under `data-source.options` to control exactly which JWT claims are forwarded to SQL Server's `sp_set_session_context`.

## Why?

Today, when `set-session-context` is `true`, DAB forwards all claims from the authenticated identity. Users have no control over which claims are sent or how nested and namespaced claims are resolved.

A simpler model is to resolve the configured claim path and store its value in `SESSION_CONTEXT` as text. Scalar values are stored as text. Arrays and objects are stored as serialized JSON text. SQL can then use `OPENJSON` and other JSON functions when needed.

## Samples

### Simple string claim

```jsonc
// JWT Payload
{ "sub": "abc123" }
```

```jsonc
// DAB Config
"session-context-claims": [
"sub"
]
```

```sql
-- SESSION_CONTEXT
sub → "abc123"
```

### Named string claim

```jsonc
// JWT Payload
{ "preferred_username": "bob" }
```

```jsonc
// DAB Config
"session-context-claims": [
"preferred_username"
]
```

```sql
-- SESSION_CONTEXT
preferred_username → "bob"
```

### Flat array claim

```jsonc
// JWT Payload
{ "roles": ["admin","writer"] }
```

```jsonc
// DAB Config
"session-context-claims": [
"roles"
]
```

```sql
-- SESSION_CONTEXT (stored as JSON text)
roles → '["admin","writer"]'
```

### Nested array claim

```jsonc
// JWT Payload
{ "realm_access": { "roles": ["editor"] } }
```

```jsonc
// DAB Config
"session-context-claims": [
"realm_access.roles"
]
```

```sql
-- SESSION_CONTEXT (stored as JSON text)
realm_access.roles → '["editor"]'
```

### Namespaced string claim

```jsonc
// JWT Payload
{ "https://example.com/tenant": "contoso" }
```

```jsonc
// DAB Config
"session-context-claims": [
"https://example.com/tenant"
]
```

```sql
-- SESSION_CONTEXT
https://example.com/tenant → "contoso"
```

### Namespaced array claim

```jsonc
// JWT Payload
{ "https://example.com/groups": ["A","B"] }
```

```jsonc
// DAB Config
"session-context-claims": [
"https://example.com/groups"
]
```

```sql
-- SESSION_CONTEXT (stored as JSON text)
https://example.com/groups → '["A","B"]'
```

### Object claim

```jsonc
// JWT Payload
{ "profile": { "name": "Bob", "region": "west" } }
```

```jsonc
// DAB Config
"session-context-claims": [
"profile"
]
```

```sql
-- SESSION_CONTEXT (stored as JSON text)
profile → '{"name":"Bob","region":"west"}'
```

## Spec: `session-context-claims`

* Type: `string[]`
* Default: `null` which means when `set-session-context` is `true`, preserve current behavior and forward all flat claims.
* Entry format: ``

* `` follows the same resolution rules as `rolesPath`: URL prefix means literal key, dot means nested traversal, otherwise literal key.

Behavior:

* Resolve the configured claim from the JWT.
* If the value is a string, number, or boolean, store its text value in `SESSION_CONTEXT`.
* If the value is an array or object, store its serialized JSON text in `SESSION_CONTEXT`.
* If the value is `null`, skip and log at `Debug`.
* SQL can parse stored JSON text using functions such as `OPENJSON`.

Notes:

* `SESSION_CONTEXT` stores scalar SQL values only. Arrays and objects are persisted as JSON text.
* Missing claim: Skip silently and do not fail the request. Log at `Debug` level: `"session-context-claims: claim '{path}' not found in JWT, skipping."`
* `SESSION_CONTEXT` key: Use the full `` string as the key name in `sp_set_session_context`.

## Backward Compatibility

| `set-session-context` | `session-context-claims` | Behavior |
| --------------------- | ------------------------ | -------------------------- |
| `false` | any | No claims forwarded |
| `true` | `null` / omitted | Forward all flat claims |
| `true` | `[]` (empty array) | Forward nothing |
| `true` | `["sub", ...]` | Forward only listed claims |

## Validation

| Case | session-context-claims | set-session-context | database-type | Result |
| ---- | -------------------------------------------- | ------------------- | ------------- | ------------------------------------------------------------------------------------------- |
| 1 | set | false | any | Startup error ("session-context-claims requires set-session-context to be true.") |
| 2 | null, empty, or whitespace entry | true | MSSQL | Startup error |
| 3 | set | true | not MSSQL | Startup warning ("session-context-claims is only supported for MSSQL.") and skip at runtime |
| 4 | null / omitted | true | MSSQL | OK (forward all flat claims) |
| 5 | empty array `[]` | true | MSSQL | OK (forward nothing) |
| 6 | valid entries | true | MSSQL | OK (forward only listed claims) |
| 7 | claim not found in JWT | true | MSSQL | Skip and log at `Debug` |
| 8 | resolved value is string, number, or boolean | true | MSSQL | Store text value |
| 9 | resolved value is array | true | MSSQL | Store serialized JSON text |
| 10 | resolved value is object | true | MSSQL | Store serialized JSON text |
| 11 | resolved value is `null` | true | MSSQL | Skip and log at `Debug` |

## Implementation

### Session Context Flow

```text
SQL query execution
→ GetSessionParamsQuery called (existing)
→ If session-context-claims is null:
→ Forward all flat claims (existing behavior)
→ If session-context-claims is non-null:
→ For each entry "path":
→ Resolve claim value (literal key / dot-path / URL key)
→ If missing: skip, log debug
→ If string, number, or boolean: convert to text and set sp_set_session_context 'path', @value
→ If array or object: serialize as JSON text and set sp_set_session_context 'path', @jsonValue
→ If null: skip and log debug
```