fix(self-packaging #62): bundled-mode HTTP serving end-to-end

[jrosskopf]

Summary

Surfaced while planning the integration test for #62: bundled-mode
HTTP serving was broken on main, not just untested. Packing
examples/ and running the result from an empty cwd crashed with:

[INFO] Bundle detected and registered (386 entries)
[INFO] Loading configuration file: "flapi.yaml"
[ERROR] Could not open file: flapi.yaml
Aborted (core dumped)

The EmbeddedArchiveFileProvider was registered globally by
detectAndRegisterEmbeddedBundle, but the config / endpoint /
template loaders bypassed it via std::ifstream and
std::filesystem::recursive_directory_iterator. All existing
self-packaging tests are CLI-level (pack, info, unpack,
idempotence, truncation fallback); nobody had ever started the
bundled binary as a server, which is exactly why #62 was deferred
in PR #56.

This PR fixes the underlying machinery and adds the three pytest
cases that were originally scoped for #62.

Changes

Provider-aware reads (src/extended_yaml_parser.cpp)

• parseFile and the static loadYamlFile now read via
  FileProviderFactory::CreateProvider(path)->ReadFile(...).
• resolveIncludePath swaps std::filesystem::exists for a
  provider-aware ConfigFileExists so {{include from ...}}
  directives find their targets inside the bundle.

Provider-aware path resolution (src/config_manager.cpp)

• parseTemplateConfig: in bundled mode, skip the
  std::filesystem::absolute() step -- the template path is a
  bundle key like sqls, not a filesystem path. Turning it into
  /cwd/sqls prevents the embedded provider from finding it.
• loadEndpointConfigsRecursively: in bundled mode, enumerate
  .yaml/.yml entries by prefix-scanning the bundle map.
  EmbeddedArchiveFileProvider::ListFiles is non-recursive by
  contract, so the walk is inlined.
• getFileProvider: in bundled mode, return a fresh provider from
  FileProviderFactory so SQLTemplateProcessor reads SQL
  templates from the bundle rather than the LocalFileProvider
  baked into ConfigLoader.

Endpoint-vs-template path collision
(src/sql_template_processor.cpp)

• getFullTemplatePath: when the endpoint parser has already
  resolved templateSource against its YAML's parent dir (which
  in bundled mode produces a full bundle key like sqls/hello.sql),
  don't re-prepend the basePath. Mirrors the existing absolute-path
  short-circuit just above; this prevented the previous
  sqls/sqls/hello.sql lookup miss.

Test (test/integration/test_self_packaging_http.py)
Three new pytest cases covering the deferred behaviours:

• Issues noticed while dockerizing the bq connection to flapi #1 unbundled flapi serves /hello with 200 + JSON body
• Ability to pass default values to query parameters #3 bundled flapi from clean cwd (with the source tree deleted)
  serves the same /hello
• Feature Request: Support Apache Arrow IPC Streaming over HTTP #9 a SQL template using read_csv('embed://data/cities.csv')
  returns the bundled CSV rows over HTTP

Test plan

• cmake --build build/release clean
• ctest -- 637 / 637 pass serially
• pytest test/integration/test_self_packaging.py test_self_packaging_http.py test_env_overrides.py -v
  -- all 17 pass (8 existing CLI-level + 3 new HTTP + 6 env overrides)
• Manual smoke: packed examples/, started from clean cwd, hit
  /hello -> 200; hit /cities reading embed://data/cities.csv
  -> 200 with 3 expected rows

Notes

• Uses --port instead of FLAPI_PORT in the new tests so this PR
  doesn't depend on feat: 12-factor env vars FLAPI_PORT + FLAPI_HOST (#63) #67 (FLAPI_PORT / FLAPI_HOST).
• clang-tidy flagged unrelated pre-existing warnings in the touched
  files (std::endl, push_back vs emplace_back, etc.); left
  untouched per the no-incidental-cleanup convention.

Closes #62.