refactor: move manifest template resolver into @microsoft/app-manifest

[iclanton]

Summary

Moves the DriverContext-free parts of fx-core's manifest templating
($[file('...')] function expansion and ${{ENV}} variable expansion) down into
@microsoft/app-manifest, so the logic can be consumed directly from the manifest
package without depending on @microsoft/teamsfx-core, a DriverContext,
localization, or UserError.

This is a behavior-preserving refactor. fx-core now delegates to the moved logic
and keeps only its host concerns (telemetry + localized error mapping).

What moves where

Relocated into @microsoft/app-manifest (single source of truth):

• expandEnvironmentVariable / getEnvironmentVariables — moved verbatim from
  fx-core/src/component/utils/common.ts.
• The file() function resolver and file reader — moved from
  fx-core/src/component/utils/envFunctionUtils.ts, decoupled from DriverContext
  and raising plain typed errors that carry the offending path/token.
• The ManifestType enum.
• Also exposes expandFileFunctionMacros (the resolution loop, returning the
  expanded content plus a function count for host telemetry) and resolveManifest,
  the host-agnostic counterpart of fx-core's getResolvedManifest.
• Reuses the package's existing strip-bom and fs-extra usage — no new deps.

Left in fx-core (host concerns only):

• common.ts re-exports expandEnvironmentVariable / getEnvironmentVariables
  from @microsoft/teamsfx-api, so all existing call sites are unchanged.
• expandVariableWithFunction delegates its loop to expandFileFunctionMacros,
  keeping the telemetry event and mapping the resolver's plain errors to localized
  UserError / fx-core FileNotFoundError via toFxError.
• getResolvedManifest is unchanged and keeps composing these primitives (its
  telemetry is preserved exactly).

Testing

• @microsoft/app-manifest unit suite: 184 passing (manifestTemplate.ts at
  100% statements / 95% branch / 100% functions).
• @microsoft/teamsfx-core: the affected suites plus every consumer of the
  re-exported env functions (aad manifest builder, common utils, scenarios,
  declarative-agent generator) pass — no regressions.
• Affected tests were updated to drive resolution through real temp files instead
  of fs mocks, since file reads now happen inside @microsoft/app-manifest.
• Lint / lint-staged clean on all changed files.

Notes / follow-ups

• Public API naming — resolveManifest, expandFileFunctionMacros, and
  processManifestFunction are new public exports on @microsoft/app-manifest.
  Happy to adjust the names/shape if the team prefers a different surface.
• Schema-validation packaging — separate from this PR, but noting it here:
  AppManifestUtils.fetchSchema is local-first (maps the manifest $schema URL to
  a bundled build/json-schemas/... file, with the network only as a fallback).
  The source bundles the schemas (src/json-schemas/, prebuild: copy-json-schema,
  files: ["build/**/*"]), but the published tarball does not include
  build/json-schemas/, so offline validation silently falls back to a network
  fetch. Fixing the publish so the schemas ship would make schema validation fully
  hermetic with no code change. Can file/fix separately if useful.
• CI — I validated the affected suites and all consumers of the re-exported
  functions locally; a full @microsoft/teamsfx-core suite run in CI is worth a
  glance (the 3 unrelated pre-existing failures need bundled template artifacts that
  a plain bootstrap doesn't produce).

Opened as a draft to gather feedback on the API surface before finalizing.

[codecov]

Codecov Report

❌ Patch coverage is 96.94656% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.08%. Comparing base (cb22bf8) to head (23a1773).
⚠️ Report is 18 commits behind head on dev.

Files with missing lines	Patch %	Lines
...es/fx-core/src/component/utils/envFunctionUtils.ts	89.47%	1 Missing and 1 partial ⚠️
packages/manifest/src/manifestTemplate.ts	98.21%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #16270      +/-   ##
==========================================
+ Coverage   86.93%   87.08%   +0.15%     
==========================================
  Files         628      629       +1     
  Lines       32732    33143     +411     
  Branches     7690     7716      +26     
==========================================
+ Hits        28454    28864     +410     
+ Misses       2440     2437       -3     
- Partials     1838     1842       +4     

Files with missing lines	Coverage Δ
packages/fx-core/src/component/utils/common.ts	76.08% <ø> (-6.46%)	⬇️
...es/fx-core/src/component/utils/envFunctionUtils.ts	89.74% <89.47%> (-2.26%)	⬇️
packages/manifest/src/manifestTemplate.ts	98.21% <98.21%> (ø)

... and 77 files with indirect coverage changes

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