Clean up clang-tidy 21 findings; move lint to ubuntu-26.04

[aaylward]

What

Follow-up to #237, which kept lint on ubuntu-24.04. The clang-tidy on 26.04 runs checks the old one didn't, and they flagged about 180 sites. This PR fixes those sites instead of turning the checks off, then moves lint to 26.04.

• modernize-use-scoped-lock: std::lock_guard<std::mutex> → std::scoped_lock, which covers most of the diff
• modernize-use-designated-initializers, -use-ranges, -use-starts-ends-with
• readability-container-contains, -math-missing-parentheses, -use-concise-preprocessor-directives
• Headers::Remove → std::erase_if. The auto-fix produced erase(ranges::remove_if(...), end()), which doesn't compile, so this one was done by hand.
• beast_transport.cc:
  • WriteWire takes const std::shared_ptr&; the lambda already copied it.
  • the TLS context is read once, right after the has_value() check (bugprone-unchecked-optional-access).
  • message::contains, which Boost 1.90 has.
• runtime/src/http/.clang-tidy also excludes clang-analyzer-optin.cplusplus.VirtualCall. It's a false positive inside Boost.Asio's resolver_thread_pool destructor, reached only from beast_transport.cc, and it sits next to the existing Boost NewDelete exclusion.
• metrics.cc aliases the pointer-to-member type so clang-format 18 and 21+ format that line the same way.

Not changed: clang-tidy 22 adds bugprone-exception-escape and -unchecked-string-to-number-conversion hits, mostly atoi in the example main() functions. The 26.04 runner's clang-tidy doesn't report them, so they're out of scope here.

Testing

• clang-tidy 21.1.6 over the lint job's file set: clean, except beast_transport.cc:158 locally. This machine has Boost 1.83 without fields::contains; the CI runner's Boost and Bazel's Boost 1.90 have it.
• clang-format 18.1.3, 21.1.8 and 22.1.8 --dry-run --Werror: clean
• bazel test //... --config=ci: 130/130 pass with both clang and gcc, under -Werror
• CI: all 16 checks green, including lint on ubuntu-26.04

Checklist

• Tests added/updated for the change (N/A: behavior-preserving refactors; existing suites cover the touched code, including Headers::Remove)
• bazel test //... passes locally
• Formatting clean (clang-format, clang-tidy)
• Architectural decisions recorded as an ADR (N/A)