Generated, compiled `Codec.typeUrl` is just TS type `string`

## Situation

Here's a difference from what Telescope generates today versus what I want from it:

```diff
diff --git a/src/codegen/circle/cctp/v1/attester.ts b/src/codegen/circle/cctp/v1/attester.ts
index 24d3c1b1dd..c5e728ae6a 100644
--- a/src/codegen/circle/cctp/v1/attester.ts
+++ b/src/codegen/circle/cctp/v1/attester.ts
@@ -26,7 +26,7 @@ function createBaseAttester(): Attester {
};
}
export const Attester = {
- typeUrl: '/circle.cctp.v1.Attester',
+ typeUrl: '/circle.cctp.v1.Attester' as const,
encode(
message: Attester,
writer: BinaryWriter = BinaryWriter.create(),
```

Without the `as const`, the Typescript compiler output is (today vs. what I want):

```diff
--- a/dist/codegen/circle/cctp/v1/attester.d.ts.orig 2025-07-11 20:52:11
+++ b/dist/codegen/circle/cctp/v1/attester.d.ts 2025-07-11 18:56:39
@@ -19,7 +19,7 @@
attester: string;
}
export declare const Attester: {
- typeUrl: string;
+ typeUrl: "/circle.cctp.v1.Attester";
encode(message: Attester, writer?: BinaryWriter): BinaryWriter;
decode(input: BinaryReader | Uint8Array, length?: number): Attester;
fromJSON(object: any): Attester;
```

## Impact

Because of that missing `as const`, an importer of the compiled files cannot use the narrow `typeUrl` constants because they're all just `string`. That's bad because it forces the user to rely on creating type registries rather than just grabbing `Codec['typeUrl']`.

## Solution

Turns out this is really easy to solve:

```diff
--- a/packages/ast/src/encoding/proto/props/index.ts
+++ b/packages/ast/src/encoding/proto/props/index.ts
@@ -3,6 +3,9 @@ import { ProtoParseContext } from '../../context';
import { ProtoType } from '@cosmology/types';
import { getAminoTypeName, getTypeUrl } from '../../amino';

+const asConst = (expr: t.Expression) =>
+ t.tSAsExpression(expr, t.tsTypeReference(t.identifier('const')));
+
export const createTypeUrlProperty = (
context: ProtoParseContext,
proto: ProtoType
@@ -11,7 +14,7 @@ export const createTypeUrlProperty = (
if (!typeUrl) return;
return t.objectProperty(
t.identifier('typeUrl'),
- t.stringLiteral(typeUrl)
+ asConst(t.stringLiteral(typeUrl))
)
};

@@ -23,6 +26,6 @@ export const createAminoTypeProperty = (
if (!str || str.startsWith('/')) return;
return t.objectProperty(
t.identifier('aminoType'),
- t.stringLiteral(str)
+ asConst(t.stringLiteral(str))
)
};
```

With this patch, I don't need anything else from you, but I thought I'd share it in case you wanted to adopt it upstream (with tests, of course).