feat: add stable response format output via optional template index

[jaredwray]

Please check if the PR fulfills these requirements

• Followed the Contributing and Code of Conduct guidelines.
• Tests for the changes have been added (for bug fixes/features) with 100% code coverage.

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature — fixes #180.

httpbin's /json output was stable between calls, which made responses comparable; mockhttp randomizes it. This PR adds the /json/1, /json/2, … endpoints suggested in the issue, applied consistently across all five content-format endpoints.

• /plain, /text, /html, /xml, and /json now accept an optional 1-based template index (e.g. /json/1 … /json/5, /plain/1 … /plain/10) that returns that fixed template, so repeated calls produce byte-identical output.
• Omitting the index keeps the existing random behavior, so /json etc. are fully backward compatible.
• An out-of-range or non-integer index returns 404 with {"error": "Template index must be between 1 and N"} rather than silently falling back to random output.
• The live timestamps embedded in the JSON and XML templates (new Date()) are now fixed literals — without this, a fixed index still wouldn't be byte-stable across requests or server replicas.
• Swagger schemas updated (index param + 404 response), so the generated API docs describe the new behavior.
• Added tests covering stable output per index, all HTTP methods, distinct templates per index, and invalid-index handling for every endpoint; coverage remains at 100% lines/functions.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Xy1LT9B3FV7ugKLR1uWodh

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7022f37) to head (0df4d22).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #181   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           41        41           
  Lines         1310      1310           
  Branches       261       261           
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  Hits          1310      1310           

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[gemini-code-assist]

Code Review

This pull request introduces support for an optional 1-based template index parameter (e.g., /:index?) across response format routes (/plain, /text, /html, /xml, /json) to allow stable, repeatable outputs. To support this, dynamic timestamps in mock templates have been replaced with static values, and JSON templates are now static objects. Schema definitions, route handlers, and comprehensive tests have been updated accordingly. Feedback on the changes suggests improving the resolveTemplateIndex helper to gracefully handle empty or whitespace-only strings (such as from trailing slashes) by falling back to a random template instead of returning a 404 error.