feat: implemented dynamic coercion for legacy tool argument aliases

- Added `normalizeSingleStringField` to dynamically map misplaced string inputs to required schema fields for single-argument tools.
- Integrated argument normalization into `validateToolArguments` to handle model-specific variations in JSON payloads during validation passes.
- Updated `coding-agent` streaming and rendering components to recognize `_input` as a legacy alias for `input` across various UI paths and logic flows.
- Refactored `hashlineEditParamsSchema` to strictly enforce the `input` field while maintaining support for legacy aliases via runtime coercion rather than schema definition.
- Corrected unit tests to reflect that `_input` is rejected by the strict schema but handled gracefully by the validation layer.

packages/coding-agent/test/core/hashline.test.ts

@@ -306,11 +306,7 @@ describe("hashlineEditParamsSchema — payload shape", () => {
 	}
 
 	it("declares only `input` as the model-facing field", () => {
-		// Create an arktype schema that mirrors hashlineEditParamsSchema structure
-		const testSchema = type({
-			input: "string",
-		});
-		const jsonSchema = getJsonSchema(testSchema) as {
+		const jsonSchema = getJsonSchema(hashlineEditParamsSchema) as {
 			properties?: Record<string, unknown>;
 			required?: string[];
 		};
@@ -327,12 +323,11 @@ describe("hashlineEditParamsSchema — payload shape", () => {
 		expect(result.success).toBe(true);
 	});
 
-	it("accepts `_input` as a provider-emitted alias for `input`", () => {
+	it("rejects `_input` as an alias for `input`", () => {
 		const result = arkSafeParse(hashlineEditParamsSchema, {
 			_input: `[x.ts]\nINS.HEAD:\n${repl("x")}`,
 		});
-		expect(result.success).toBe(true);
-		if (result.success) expect(result.data.input).toBe(`[x.ts]\nINS.HEAD:\n${repl("x")}`);
+		expect(result.success).toBe(false);
 	});
 
 	it("still requires `input`", () => {