fix(conda): preserve gzip for large repodata indexes

[abhinavgautam01]

Closes #321

Summary

Request gzip for Conda JSON repodata and preserve the compressed response through caching and delivery. This reduces transfer size and lets large indexes fit within the metadata cache limit when their compressed representation fits.

Changes

• Request gzip for repodata.json and current_repodata.json.
• Cache and replay compressed bytes with the correct Content-Encoding and Content-Length.
• Preserve gzip for uncached streaming and offline cache fallback.
• Keep repodata.json.bz2 on identity encoding.
• Decode gzip before cooldown filtering, rejecting corrupt or truncated gzip responses.
• Retain metadata size limits, including the decoded-size limit for cooldown filtering.

Scope and limitations

• Encoding changes are limited to Conda JSON repodata; other ecosystems remain unchanged.
• Upstreams that return identity responses remain supported, subject to the existing size limit.
• Cooldown filtering still requires the decoded JSON to fit within metadata_max_size.
• HTTP timeout settings remain unchanged.

Validation

Added regression coverage for both JSON routes, streaming, fresh-cache hits, offline fallback, identity responses, .bz2 preservation, size limits, cooldown filtering and invalid gzip.

Confirmed the new regression tests reproduce the failures against the original handler.

Local checks passed:

• Formatting and git diff --check
• go tool golangci-lint run ./... — 0 issues
• go test -race ./...
• go test ./internal/handler -run 'TestConda' -count=10
• go vet ./...
• go build ./...
• go mod tidy -diff

[andrew]

Matches the fix specified in the issue and follows the proxyCachedWithEncoding pattern already used for the Homebrew API. Good catch on the cooldown path: handleRepodata was already sending Accept-Encoding: gzip and handing the compressed body to ReadMetadata, so filtering silently fell through to writing gzip bytes under Content-Type: application/json. The decode fixes that, and ReadMetadata's limit keeps the decompressed size bounded.