bedrock.WithLoadDefaultConfig panics on ordinary AWS config failures (unknown profile, no SSO cache), so a CLI answers a login prompt with a stack trace

### Summary

`bedrock.WithLoadDefaultConfig` panics when `config.LoadDefaultConfig` returns an error:

```go
func WithLoadDefaultConfig(ctx context.Context, optFns ...func(*config.LoadOptions) error) option.RequestOption {
cfg, err := config.LoadDefaultConfig(ctx, optFns...)
if err != nil {
panic(err)
}
return WithConfig(cfg)
}
```

`bedrock/bedrock.go:186-192` (v1.55.1)

Every failure this converts into a panic is an ordinary, recoverable, user-caused condition rather than a programmer error: a profile name that does not exist, a missing or malformed `~/.aws/config`, an SSO cache that was never populated. For a CLI those are the common case on first run, so the tool answers "you have not logged in yet" with a stack trace, and the user cannot tell a misconfiguration from a crash.

### Versions

- `github.com/anthropics/anthropic-sdk-go v1.55.1`
- `github.com/aws/aws-sdk-go-v2/config v1.32.34`
- go1.25.6, darwin/arm64 (not platform-specific)

### Reproduction

No AWS setup required beyond a profile name that does not exist:

```go
package main

import (
"context"
"fmt"

"github.com/anthropics/anthropic-sdk-go"
"github.com/anthropics/anthropic-sdk-go/bedrock"
)

func main() {
defer func() {
if r := recover(); r != nil {
fmt.Printf("PANICKED: %v\n", r)
}
}()
_ = anthropic.NewClient(bedrock.WithLoadDefaultConfig(context.Background()))
fmt.Println("no panic")
}
```

```console
$ AWS_PROFILE=definitely-not-a-real-profile AWS_REGION=us-east-1 go run .
PANICKED: failed to get shared config profile, definitely-not-a-real-profile
```

Without the `recover`, that is a stack trace and a non-zero exit.

### Expected

The failure is surfaced as an error the caller can present, not a panic.

### Why it is awkward to fix, and two options

`option.RequestOption` has no error channel, which is presumably why the panic is there. Two ways out, either of which would work for us:

1. **An erroring variant**, leaving the current function untouched for callers who genuinely want to fail fast:

```go
func WithLoadDefaultConfigErr(ctx context.Context, optFns ...func(*config.LoadOptions) error) (option.RequestOption, error)
```

2. **Defer the failure to the first request**: capture the error in the returned option and have the middleware return it, so a caller that never issues a request never sees it and one that does gets a normal `error`.

### Workaround

Call `config.LoadDefaultConfig` directly, keep the error, and pass the config to `bedrock.WithConfig` — deferring the failure to the first request with a message naming the likely fix. That is what we do in [alibaba/open-code-review](https://github.com/alibaba/open-code-review), and it is the only reason an expired SSO session there produces a sentence instead of a stack trace.

Note for anyone landing here from the same direction: `bedrock.WithConfig` needs its own care on SSO profiles — see #414 and #415.