fix(postgrest): replace SCREAMING_SNAKE_CASE HTTP method constants with enum

[grdsdev]

Summary

• Replaces six file-private METHOD_* string constants with a _HttpMethod enum that exposes a .value getter returning the uppercase string
• Removes the // ignore_for_file: constant_identifier_names suppression
• Removes the _Nullable<T> typedef, replacing its single use with a _typeOf<T>() helper for runtime nullable type comparison

Changes

• postgrest_builder.dart: Adds _HttpMethod enum and _typeOf<T>() helper; removes constants, typedef, and lint suppression; updates all internal usages
• postgrest_query_builder.dart: Updates method: arguments from string constants to enum values
• postgrest_rpc_builder.dart: Same
• postgrest_transform_builder.dart: Same

Root Cause

postgrest_builder.dart used SCREAMING_SNAKE_CASE top-level constants to represent HTTP method strings, requiring a file-level lint suppression (constant_identifier_names). These are private to the library and have no public API impact.

Testing

• dart analyze — no issues
• dart test --name "Custom http client" — 4/4 pass (integration tests require a live PostgREST server not available in CI without Docker)

Acceptance Criteria

• constant_identifier_names lint passes with no file-level suppression
• _Nullable typedef removed
• No public API change (all changes are private)
• All available tests pass

Linear Issue

Closes: SDK-810

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🤖 Generated with Claude Code /take

[Vinzent03]

I'm really unsure about this change. I implemented that and was a strange required hack to work with nullability and generics. The tests do pass, but I'm not totally sure that case is covered. Is there any specific reason to change this here?

[grdsdev]

Reverted the _Nullable → _typeOf change — the typedef is back as-is. The PR now only replaces the METHOD_* constants with the _HttpMethod enum. @Vinzent03 could you take another look?