Fix CodeUnits write ambiguity + 500 fallback after deferred startwrite (#1302, #1303)

[quinnj]

Fixes #1302, fixes #1303 (both reported by @PingoLee — thanks for the unusually precise root-cause analysis on both).

#1302 — write(::Stream, ::Base.CodeUnits) ambiguity

Base.CodeUnits <: DenseVector{UInt8}, so Base's write(::IO, ::Base.CodeUnits) was ambiguous with the StridedVector{UInt8} methods on Stream (and on the internal _RequestDeadlineWriteIO). Added disambiguating CodeUnits{UInt8} methods delegating to the existing byte paths. Verified through a real handler (write(stream, codeunits("ok")) → client receives ok) and a client HTTP.open body write.

#1303 — handler throw after startwrite silently dropped the connection

Exactly as the issue diagnosed: h1 FIXED mode defers the response head at startwrite while flipping response_started, and the error path gated its 500 fallback on !response_started — so a post-startwrite throw tore down the connection with zero bytes on the wire (ParseError: unexpected EOF client-side).

Fix: a new head_committed flag on Stream, set only at the two places head bytes actually reach the transport (h1 buffered write, h2 HEADERS frame). The h1 error path now answers with a raw 500 (_try_write_server_error!, same as sibling branches) when the handler fails with response_started set but nothing committed. Once the head is on the wire (e.g. chunked mode writes it at startwrite), behavior is unchanged — the control test asserts no second head/spurious 500 is emitted.

scenario	before	after
throw after startwrite, FIXED (deferred head)	connection drop, client ParseError: unexpected EOF	500 Internal Server Error
throw after startwrite, chunked (head committed)	connection close	unchanged

Tests

3 new testsets in http_server_http1_tests.jl (codeunits round-trip, deferred-startwrite → 500, committed-head control). Locally: h1 189/189, client 472/472, core 162/162.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.03%. Comparing base (94c71fe) to head (80a72a8).

Files with missing lines	Patch %	Lines
src/http_transport.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1305      +/-   ##
==========================================
+ Coverage   87.02%   87.03%   +0.01%     
==========================================
  Files          28       28              
  Lines       11017    11026       +9     
==========================================
+ Hits         9587     9597      +10     
+ Misses       1430     1429       -1     

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