…roke footers to be properly recognized (#2388)

The original logic expect footer to follow `key: value` (exactly one
space before value) format as conventionalcommit specs. For multi-line
footers, when parsing
([logic](https://github.com/googleapis/librarian/blob/9b28aa4a3619579dfa59c430afa612589b353598/internal/conventionalcommits/conventional_commits.go#L336)),
the line content "Source-Link:" would fail the regex match and thus not
properly recognized as footer, and gets appended to the value of the
previous footer.

By relaxing the regex a bit to allow any number of spaces before "value"
in footer, the multiline footer can be recognized.

note: this format below from owlbot PR
([example](googleapis/google-cloud-python@34a7916))
is technically not valid footer according to conventionalcommits.org
spec
[here](https://www.conventionalcommits.org/en/v1.0.0/#:~:text=Each%20footer%20MUST%20consist%20of%20a%20word%20token%2C%20followed%20by%20either%20a%20%3A%3Cspace%3E%20or%20%3Cspace%3E%23%20separator%2C%20followed%20by%20a%20string%20value).
But we are relaxing to allow it for onboarding libraries.
```
Source-Link:
googleapis/googleapis@d300b15
```

This should solve the first issue causing this bug, see
[comment](#2080 (comment))

For #2080

Librarian Version: v0.0.0-20251009012716-e267b1dfc9b9
Language Image:
us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/librarian-release-container:latest
<details><summary>librarian: 0.4.0</summary>

##
[0.4.0](v0.3.0...v0.4.0)
(2025-10-09)

### Features

* add `git log` functionality (#2510)
([e267b1d](e267b1d))

* Modify README.md to match our agreed-upon wording (#2503)
([2dc6fc6](2dc6fc6))

* Show actual enum values on setter samples. (#2491)
([13fd5a1](13fd5a1))

* sort library state when writing to `state.yaml` (#2504)
([35e4495](35e4495))

* Generate methods that use server-side streaming (#2474)
([31a496d](31a496d))

* allow configuring tag_format in config.yaml (#2236)
([e0eeddb](e0eeddb))

* `update` supports different roots (#2473)
([65fa7ed](65fa7ed))

* Support git push via SSH (#2397)
([7259194](7259194))

* mount output dir in configure docker command (#2439)
([365019e](365019e))

* Update templates for otel (#2443)
([ea54ba6](ea54ba6))

* support aggregated pagination (#2432)
([5b010b9](5b010b9))

* override pagination items field (#2441)
([4a3664a](4a3664a))

* include source root in configure request (#2431)
([475a62d](475a62d))

* rust&#43;disco bytes (de)serialization (#2428)
([b2a186c](b2a186c))

* enum serialization for non-proto sources (#2410)
([516f29e](516f29e))

* optional singular fields in discovery (#2408)
([c86c2db](c86c2db))

* deprecated elements in discovery docs (#2399)
([1d83853](1d83853))

* allow &#34;.&#34; as valid `source_roots` entry (#2396)
([2f52aa7](2f52aa7))

* inline messages in discovery docs (#2394)
([043cf65](043cf65))

* write pr-body.txt to the work root when not pushing (#2395)
([862c7d7](862c7d7))

* define LIBRARIAN_GITHUB_TOKEN as a constant (#2367)
([5979bfd](5979bfd))

### Bug Fixes

* clarify doc and comment with commit and push flag (#2507)
([645e42a](645e42a))

* keep first value for repeated footer keys (#2440)
([e51490d](e51490d))

* group commit msg if they have the same piper id and subject (#2496)
([60b7d38](60b7d38))

* real runs for `rust-publish` (#2484)
([15d63c2](15d63c2))

* Remove unnecessary dereference in transport template (#2481)
([610cec9](610cec9))

* shorten SHAs in generate commit msgs (#2472)
([de336b1](de336b1))

* handle slashes in library IDs (#2463)
([91c0189](91c0189))

* nil check for github.NewClient (#2465)
([71db17e](71db17e))

* Conventional Commit parser filters files based on Librarian flow
(#2433)
([6cb55d2](6cb55d2))

* Ensure detailed-tracing-attributes flag is fully propagated (#2438)
([a184f0d](a184f0d))

* relax footer regex to allow line broke footers to be properly
recognized (#2388)
([dcbe063](dcbe063))

* release init provides read-only full repository (#2370)
([2e11004](2e11004))

* write the PR body on &#34;commit but no push&#34; (#2430)
([88698b4](88698b4))

* should not push to github when no push flag specified (#2405)
([2a49ae6](2a49ae6))

* only mount global files in configure command (#2377)
([1a2aaa0](1a2aaa0))

* `Map` field attribute in discovery (#2414)
([00c645f](00c645f))

* missing IDs in discovery doc fields (#2386)
([d579dd7](d579dd7))

</details>