feat(headers): flip CLAUDE_CODE_ENTRYPOINT default from 'cli' to 'sdk-cli'

Proxyman captures of Claude Code 2.1.109+ show the default entrypoint is
'sdk-cli', not 'cli'. Setting the plugin default to match removes the
last remaining divergence in the billing header and User-Agent when
CLAUDE_CODE_ENTRYPOINT is unset.

Source
- src/headers/billing.ts: cc_entrypoint default flipped in nullish
  coalescing fallback.
- src/headers/user-agent.ts: getClaudeEntrypoint default flipped in
  logical OR fallback.

Tests
- tests/regression/headers/billing-edge-cases.test.ts: updates the
  unset-default assertion to expect 'sdk-cli' and refreshes the '??'-vs-'||'
  comment accordingly.
- tests/regression/fingerprint/cc-comparison.test.ts: rewrites the
  top-level describe block to compare against the current fallback
  fingerprint (instead of the retired 2.1.107 pin), uses the runtime
  profile from getRequestProfile for user-agent assertions, and adds
  three new regression tests:
    1. signature mode filters inbound anthropic-beta entries while
       keeping oauth-2025-04-20.
    2. plain-text mention of 'user:file_upload' does not trigger the
       Files API beta.
    3. signature mode overrides inbound 'accept' and strips
       'x-session-affinity'.

12/12 scoped tests pass.