chore(spanner): implement key_recipe routing engine for basic types

[olavloite]

Add key_recipe location-aware routing engine (encode_key_from_recipe and encode_key_from_recipe_into) to generate binary storage specification keys (ssformat) from Spanner KeyRecipe metadata for location-aware routing.

[gemini-code-assist]

Code Review

This pull request introduces a key recipe evaluation engine for Spanner location-aware routing, enabling the encoding of KeyRecipe specifications into lexicographical binary keys, along with unit and golden conformance tests. It also refactors composite tag validation in ssformat.rs to use u32 instead of i32. The review feedback identifies a correctness issue in the golden tests' byte unescaping logic where casting char to u8 can truncate non-ASCII characters, and suggests applying the 'Return Early' style guide principle in check_supported_key_type and encode_bytes_part to reduce nesting.

[codecov]

Codecov Report

❌ Patch coverage is 99.35897% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.27%. Comparing base (35b9590) to head (d9f6f45).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spanner/src/routing/key_recipe.rs	99.35%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6281      +/-   ##
==========================================
+ Coverage   96.24%   96.27%   +0.02%     
==========================================
  Files         280      281       +1     
  Lines       72146    72767     +621     
==========================================
+ Hits        69440    70058     +618     
- Misses       2706     2709       +3     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a key recipe evaluation engine for Spanner location-aware routing, implementing key encoding from a KeyRecipe and column values, along with extensive unit and golden conformance tests. It also updates composite tag constants and validation in ssformat.rs to use u32 instead of i32. The review feedback suggests enhancing transactional safety in encode_key_from_recipe_into by truncating the buffer on error, and refactoring check_supported_key_type and encode_part to return and use &TypeCode directly to avoid redundant lookups.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a key recipe evaluation engine for Spanner location-aware routing. It adds key_recipe.rs to evaluate and encode KeyRecipe instances into lexicographical binary keys, along with a comprehensive suite of unit tests and golden conformance tests in golden_tests.rs. Additionally, it updates composite tag validation constants and the append_composite_tag function in ssformat.rs to use u32 instead of i32. I have no further feedback to provide.