fix: reject invalid rendering proxy header payloads

[kitsuyui]

Summary

• reject X-Rendering-Proxy payloads that are valid JSON but not JSON objects
• reject request header fields whose types do not match the documented protocol schema
• document the 400 Bad Request behavior for invalid request payload shapes

Testing

• bun run format
• bun run lint
• bun run typecheck
• bun run test -- src/server/request_options.spec.ts src/server/index.spec.ts
• bun run build
• pre-push hook: bun run lint
• pre-push hook: bun run typecheck
• pre-push hook: bun run test

[github-actions]

Code Metrics Report

	main (de60997)	#1657 (df8ece0)	+/-
Coverage	84.1%	84.3%	+0.1%
Code to Test Ratio	1:0.5	1:0.5	-0.1
Test Execution Time	11s	9s	-2s

Details

  |                     | main (de60997) | #1657 (df8ece0) |  +/-  |
  |---------------------|----------------|-----------------|-------|
+ | Coverage            |          84.1% |           84.3% | +0.1% |
  |   Files             |             11 |              11 |     0 |
  |   Lines             |            234 |             243 |    +9 |
+ |   Covered           |            197 |             205 |    +8 |
- | Code to Test Ratio  |          1:0.5 |           1:0.5 |  -0.1 |
  |   Code              |           1775 |            1827 |   +52 |
+ |   Test              |           1003 |            1014 |   +11 |
+ | Test Execution Time |            11s |              9s |   -2s |

Code coverage of files in pull request scope (72.9% → 74.6%)

Files	Coverage	+/-	Status
src/server/index.ts	66.6%	0.0%	modified
src/server/request_options.ts	95.6%	-4.4%	modified

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Reported by octocov